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How to Plant Onions: Ultimate Guide

If you’re planning on adding onions to your next season’s garden strategy, there’re some few facts you’ll need to know about planting and harvesting them.

Some varieties will grow better in various regions and climates. While these are general guidelines, reading about specific types for your area is best for a good outcome. 

Below is a basic guide on how to plant onions. 

How Long Do Onions Take To Grow? 

They go through a variety of stages in their growth lifecycle that will span months. From germination at one week to about a month for the seeds, until maturity at five months.

Due to the lengthy growth period, there are ways to ensure they are planted to dodge troublesome weather and provide the best outcomes.

Simply put, an onion plant can take 100 to 175 days to reach the harvesting stage after planting, depending on the variety.

When deciding to plant from seed, it’s best to start them indoors in a tray or container. Onion sets, which are further along in the process, maybe another option.

Planting the Seeds 

Start seeds in a container about 6-10 weeks before the new sprouts will be ready for the outside world. A shallow container with dampened soil that is not overly soggy is required.

Select varieties that are right for your region and follow the package as instructions may vary by climate and variety.

Seeds should be retained in a warm and moist area until they sprout and are mature enough for transplantation. 

Planting From Sets 

Onion sets are miniature bulbs explicitly cultivated for gardening that, once put into the soil, will develop to full-size bulbs after three and a half months approximately.

These are a bit more robust and grow much faster than seeds. Additionally, due to the full lifecycle, these reduce the need to plant when freezing and other non-conducive planting conditions may prevail.

When picking up bulbs at a nursery, you should check that they are dry (typically bought in mesh bags) and firm.

Checking for leaf and bulb health before planting is essential for the eventual output of the mature plant. 

See also: How to Plant Garlic: A Basic Growing Guide


Spacing will be dependent on if planting from seeds, sets, or transplanting.

If going for seeds, they should be started six weeks before ground planting, in containers, and then spaced approximately four to five inches apart.

When doing sets, these should be planted about two to six inches apart.

Straw mulch should be added between sets, and in between the rows, allow for 12 to 18 inches of spacing. 

Planting Depth 

Seeds should be sunk only about half an inch below the soil level.

First, it is crucial to amend the soil with compost and other organic material to support the onion’s growth.

If planting from sets, then they should be placed approximately one inch down into the amended soil where they can take root.

Remember, the type of onion will also determine how deep you’ll put it in the ground. It is crucial to know your climate and variety to fully understand its growing needs. 

Water Requirements

This is a vegetable that needs a substantial amount of water.

It is best to utilize mulch around the plants to regulate water and provide about an inch a week. The mulch ensures proper saturation without making the earthy too soggy around these plants.

Well-drained soil is necessary for these plants to best mature.

Rot is a real consideration, which is why ensuring the best mulch or moisture barriers without drowning the plant is critical. 

Growing From Scraps 

It is possible to grow onions from onions.

  • Cut off the bottom and put toothpicks or other materials that can hold the scrap in place over a cup or bowl.
  • Fill the dish, jar, or container with water and suspend the scraps above and water source.
  • Soon you’ll start to see tiny sprouts coming out the top and root-like tentacles from the bottom.
  • Place the sprouted scrap into the ground or a pot, covering until just the sprouts are above the soil line.
  • Water, drain, and cultivate as you would any other onion grown from seed or set.

Growing in Containers 

They can be grown in shallow containers of at least eight inches, either from sets or seeds.

  • Fill the container about halfway with soil, and keep it watered but well-drained once planted.
  • Adding a nitrogen-rich compost until the bulbs appear and if using mulch, then regular but not overwatering.
  • The bulb should remain at least an inch under the soil, and if exposed, should be recovered.
  • Leaves can be cut down to help the plant to focus more fully on the development of the bulbs.

Companion Plants 

Anything in the cabbage family makes a great companion plant for onions.

They help to deter aphids, Japanese beetles, and rabbits, so anything that these pests enjoy can be married with the onion.

Additionally, lettuce and carrots, for instance, have very different root systems that won’t compete for the same resources under the soil.

Tomatoes, lettuce, and strawberries, to name just a few other examples, are great companion plants.

Strawberries, for instance, benefit from being close to onions, which help deter diseases that impact that plant.

Avoid all varieties of peas and beans as there is a chemical incompatibility with these vegetable families.

Finally, chamomile is a plant that can actually help to improve the overall flavor, so that’s a great companion plant to add to your garden. 

How Do You Know When Onions Are Ready? 

Top green leaves that start to wilt is the first sign that they are near harvesting ready.

The bulbs will continue into maturity as the leaves start to lose their color. This occurrence is due to the plant now putting all reserves toward the aging of the bulb itself.

If any have flowers on the stalks, they should be harvested immediately. 


Dig or pull on the tops to safely remove the whole plant from the soil. Next, shake the dirt off from around the bulbs and ready them to be cleaned and stored.

It is recommended to let the bulbs sit in the garden for 1-2 days to allow natural drying.

Additionally, you may wish to loosen the dirt and bring it back from the bulb to allow drying before harvesting.

Curing by allowing them to dry will be evident as their outer layers tighten around the bulb and dry.

There are two common varieties — pungent and mild. They may differ in the length of time that they can be kept before going off.

Pungent has a much longer time before re-sprouting or going bad. 

Pest and Diseases  

  • Onion fly is one of the most prolific pests that affect the entire allium family. These lay eggs and hatch maggots that feed upon the seedlings.
  • This fly resembles the house fly. After 3 days of their eggs being laid, they mature and burrow their way into the onion.
  • The lesser bulb fly looks similar but is smaller. Once infected by them, the plants must be dug up and destroyed.
  • Thrips are another insect that lay eggs on the leaves and then suck the sap from them dry.

There may be many diseases also due to water not controlled cause lesions and rot signature on the plant.

Diseases include neck rot, purple blotch, botrytis leaf blight, damping off, downy mildew, green mold, and more. 


First and foremost, onions must be stored in dry and cool locations. If too moist or warm, resprouting can occur.

Onions sprouted with green in the center when cut into, still can be eaten in the early stages.

Mesh bags, bushel baskets, or blat cardboard boxes with holes provide the best storage methods to keep them dry and cool for the best storage in the short term.

Keep in mind that onions will only last about seven to ten days before thoroughly drying out or resprouting even in the best of conditions.

This is true in most mild varieties but may be extended weeks or even months in proper conditions for pungent types.

Again regional, climate changes, and your storage options should be considered for the best outcome of your crop.


Now, you are prepared to make a decision on planting onions in your garden. Whether you will start the seeds six to ten weeks early or use the small bulbs of sets will help determine the length of growth.

Full gestations from seed to maturity is five months and requires consistent but well-drained water access.

The leaves will stay green and be one of the best indicators of good health until just before harvesting when they will turn yellow and start to droop.

Additionally, cool storage and dry conditions will be needed until you can, cut, or otherwise enjoy your harvest.

Armed with all the relevant information, you can now lace onions into your garden.

Sasha Brown

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